1. Field of the Invention
The present invention relates to a process for forming a polymeric solution or gel having a viscosity of less than about 50,000 cps which includes the steps of forming a solvent system of an organic liquid and a polar cosolvent, the polar cosolvent being less than about 15 wt. % of the solvent system, a viscosity of the solvent system being less than about 1,000 cps: dissolving a neutralized sulfonated polymer in the solvent system to form a solution, a concentration of the neutralized sulfonated polymer in the solution being about 0.5 to about 20 wt. %, a viscosity of the solution being about 5 to about 5,000 cps; and adding about 1 to about 500 vol. % water to the solution having a viscosity less than about 5,000 cps, the water being immiscible with the solution and the polar cosolvent transferring from the solution phase to the water phase thereby causing the viscosity of said solution to increase by a factor of at least 2 to less than 50,000 cps.
2. Description of the Prior Art
There are many applications for very viscous or gelled solution of polymers in organic liquids which are quite diverse. There are also a number of physical and chemical techniques for preparing such systems. The present invention is concerned with a process for converting a relatively low viscosity organic liquid solution of an ionic polymer into a viscous or gelled system via a rapid process which under certain conditions can be reversed. The potential applications for this process and the products derived therefrom will be evident in the instant application.
There are major problems in the direct preparation of polymer solutions or gels via conventional techniques such as polymer dissolution. For example, attempts to form a high viscosity (&gt;500,000 cps) solution of polystyrene in a suitable solvent such as xylene can be difficult. The levels of polymer required are either very high (20 to 50 wt. % concentration) or the molecular weight of the polymer must be extremely high. In either event the dissolution process is extremely slow even at elevated temperatures, and even then it is difficult to achieve homogeneous polymer solutions free of local concentrations of undissolved, or poorly dissolved polymer. Thus, the process of achieving such solutions can be difficult and the concentration of polymer in the solution to achieve high viscosities can be uneconomically high.
These are various chemical approaches to the solution of the problems outlined above, that is polymer chain lengthening reactions which can occur to give viscous solutions such as by the reaction of hydroxyl terminated polymers with diisocyanates etc. Such processes have inherent disadvantages which preclude their use in the intended applications of this invention.
The instant invention describes a process which permits (1) the preparation of polymer solutions of sulfonated polymers in organic liquid having reasonably low viscosities (i.e., less than about 5,000 cps), (2) the preparation of viscous solutions or gels from such solutions by the simple process of mixing water with the polymer solution and (3) the reversion of such viscous solutions or gels to relatively low viscosity mixtures by the reincorporation of polar cosolvents which are water immiscible at a desired stage. These operations are achieved by the use of appropriate concentration of polymers having low concentrations of ionic groups present, preferably metal sulfonate groups. Such polymers are described in detail in a number of U.S. Pat. Nos. (3,836,511; 3,870,841; 3,478,854; 3,642,728; 3,921,021) which are herein incorporated by reference. These polymers possess unusual solution characteristics some of which are described in U.S. Pat. No. 3,931,021. Specifically such polymers such as lightly sulfonated polystyrene containing about 2 mole % sodium sulfonate pendant to the aromatic groups are typically not soluble in solvents commonly employed for polystyrene itself. However, the incorporation of modest levels of polar cosolvents permit the rapid dissolution of such ionic polymers to form homogeneous solutions of moderate viscosity.
In the instant process, the role of the polar cosolvent is that of solvating the ionic groups while the main body of the solvent interacts with the polymer backbone. For example, xylene is an excellent solvent for the polystyrene backbone and when combined with 5% methanol readily and rapidly will dissolve the previous example of lightly sulfonated polystyrene.
The remarkable and surprising discovery of the instant invention pertains to the following observation.
When small (or large) amounts of water are combined and mixed with solutions of ionic polymers dissolved in such mixed solvent systems as those described above, it is possible to convert such low viscosity systems into more viscous gels or solutions. Indeed it is possible to achieve increases in viscosity by factors of 10.sup.4 (10,000) or more by the addition of only 5 to 15% water based on the polymer solution volume.
This unusual behavior is postulated to arise from the removal of the polar cosolvent from the polymer solution phase into a separate aqueous phase. Consequently, when this occurs the physical crosslinking of the ionic groups is again manifested resulting in an increase in solution viscosity. While this postulate is consistent with available data it is intended that this invention not be limited by this operating hypothesis. The resulting gels or thick solutions appear quite homogeneous.